This invention relates generally to variable-ratio belt-driven transmissions and more specificially to belt-driven transmissions for which the ratio of belt velocity to the angular velocity of the transmission output is continuously variable.
A belt-driven transmission converts power supplied by an endless belt to rotary power. It is desirable for many applications that the transmission ratio, the ratio of the linear velocity of the belt to the angular velocity of the transmission output, be not only variable but continuously variable.
Perhaps the oldest and simplest variable-ratio transmission is an assembly of pulleys of different diameters on a common hub. The transmission ratio is changed by shifting the belt from one pulley to another.
A variable-ratio transmission for use with V-belts consists of a pulley split into two halves with a means for varying the separation of the two halves. Since the cross section of the split-pulley belt channel is V-shaped to match the sides of the V-belt, the distance of the V-belt from the axis of rotation and the transmission ratio is proportional to the separation of the pulley halves. The split-pulley approach has the advantage of having a transmission ratio that is continuously variable. Unfortunately, the torque available from such a transmission is limited since the torque depends on the frictional force that maintains the V-belt in contact with the pulley.
In order to obtain higher torques, it is necessary to use corrugated belts and pulleys (or chains and sprockets) that provide positive, non-slip engagements. A transmission with a continuously-variable transmission ratio and based on the use of a corrugated belt and a matching corrugated pulley implies a pulley having a continuously-variable diameter and a constant rib pitch. Such a pulley tends to be very complex. A need exists for a simple continuously-variable transmission that is suitable for high-torque applications.